I think you can do it by putting a resistor across lugs 1 and 3 of the volume pot.
You'd have to first measure the pot where it is at unity gain, taking the resistance of the pot between lugs 2 and 3. Then... you'd compute the resistor value to use across lugs 1 and 3, such that it when put in parallel with the 10K pot, it equals the value between 2 and 3 at unity gain.
For example, if you measured the resistance between 2 and 3 at unity gain at 8K, then you'd want to put a 40K resistor across lugs 1 and 3, so when you put the pot at 0, the resistance would be 8K (the value of 10K and 40K in parallel).
I think that would work but maybe there are better ways.